This application claims priority under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/EP99/00350, filed Jan. 20, 1999, which claims priority to Austrian Application No. 140/98 filed Jan. 28, 1998.
This invention relates to a process for treating circuit water from a paint shop from which the paint washed out from the waste air of the paint shop is at least periodically removed.
In order to be able to recover the excess paint which accumulates in paint shops and which is discharged with the waste air from the painting cubicle and washed out from the waste air, it is known (EP 0 675 080 A2) that the paint-laden washing water can be subjected to a multistage membrane filtration process. As a result of this membrane filtration process, the paint removed largely corresponds in its composition and concentration to the waterborne paint used in this known paint shop while the treated, i.e. clean, water is recirculated to the paint shop, so that a water circuit is established via the membrane filtration stage. Unfortunately, the disadvantage of this known process for removing the excess paint washed out from the circuit water is that the paint particles tend to stick to one another and then harden. As a result, paint transported by the circuit water is partly deposited in the pipes and pumps which gradually become blocked. Similarly, paint can also be deposited in the waste air system of the paint shop, with the result that not only are maintenance costs comparatively high, operational problems can also be expected to occur as a result of the paint deposits. In addition, paint consumption can increase and losses of quality can occur.
Paint particles can be prevented from, sticking to one another by using coagulating agents. However, these coagulating agents, which consist of a wax, a polyelectrolyte or a clay product, are filtered out with the paint so that the coagulating agent used has to be continuously supplied in a considerable concentration. Despite the use of coagulating agents, however, the deposition of paint particles and coagulate residues cannot be prevented.
Accordingly, the problem addressed by the present invention was to design a process for treating circuit water from paint shops of the type mentioned at the beginning in such a way that paint particles would be safely prevented from sticking to one another, thus providing for low-maintenance and largely trouble-free operation.
According to the invention, the solution to the problem stated above is characterized in that polyaspartic acid is added to the circuit water.
The novel use of polyaspartic acid, which is produced in known manner (DE 43 05 368 A1, DE 43 06 412 A1, DE 43 10 503 A1, DE 43 23 191 A1, DE 44 27 233 A1, DE 44 28 639 A1) and which is otherwise used above all as a detergent additive, surprisingly prevents paint particles from sticking to one another, so that pipes, pumps or other parts of the paint shop equipment which come into contact with the paint-laden circuit water remain free from deposits. Since, in addition, the polyaspartic acid remains dissolved in the water and, accordingly, is circulated with the circuit water, it is merely necessary to replace the polyaspartic acid lost either through the evaporation of water or with the very small amount of water removed with the paint. There is presumably no need to place any particular emphasis on the fact that any auxiliaries required, such as solubilizers, wetting agents, corrosion inhibitors or biocides andxe2x80x94particularly in the case of waterborne paintsxe2x80x94foam inhibitors, may be added to the circuit water with the polyaspartic acid.